The present invention relates to simultaneous production of alkali metal or ammonium peroxodisulphate salts and alkali metal hydroxide using a continuous-action electrochemical process, in the electrolytic phase whereof alkali metal sulphate is electrodialysed in a three-space electrolytic cell divided by an anion and cation exchange membrane.
The main products obtained with the method of the present invention, that is, inorganic peroxodisulphate compounds, are powerful oxidizers, as is well known in the art, but far more specific compared, for instance, with hydrogen peroxide. They are used, inter alia, for purifying metals and etching, and as initiators in polymerizing reactions. The commercial production of peroxodisulphate salts takes place exclusively by means of electrolysis.
The overall reaction of the electrolytic phase of the electrochemical production processes of alkali or ammonium peroxodisulphates known in the art, consisting of oxidation of sulphate ions with anode and from the hydrogen development reaction by means of a cathode, may be presented in the following form: EQU M.sub.2 SO.sub.4 +H.sub.2 SO.sub.4 .fwdarw.M.sub.2 S.sub.2 O.sub.8 +H.sub.2
where M is an alkali metal ion or an ammonium ion. Subsequent to the electrolysis, the anolyte and the catholyte are partly combined. From the solution thus obtained peroxodisulphate salt is obtained as a product by means of crystallisation. The electrolysis is typically performed in a two-space cell in which the anode and cathode spaces have been separated by a porous membrane or diaphragm. The function of the porous membrane is to avoid the travelling of the peroxodisulphate ion produced in the anode to the cathode by preventing the solutions in the anode and the cathode space from being mixed mechanically.
The significance of the other main products of the process according to the present invention, the alkali metal hydroxides, the production of sodium hydroxide of which is clearly greatest in volume, is great in the chemical and wood-processing industries. Today, nearly all commercially produced lye is produced electrochemically by a chloride--alkali process, the total reaction of the electrolytic phase whereof being as follows: EQU 2NaCl+2H.sub.2 O.fwdarw.Cl.sub.2 +2NaOH+H.sub.2
Because of the decreasing demand of chloride, it is important to develop new substitutive electrochemical or chemical production methods for lye.
From the point of view of the economical factors related to electrochemical syntheses, it is often essential that commercially utilizable produces are produced in both electrode reactions. In such instances, cell structures are usually used in which the solutions in the anode space and in the cathode space and in a potential supply space of the cell are separated from one another. The partial separation of the spaces was earlier performed merely by means of porous diaphragms inhibiting merely mechanical admixing, whereas the spaces are nowadays most often separated with the aid of ion exchange membranes affecting selectively the travelling of the ions.
As an example of the use of ion exchange membranes in producing peroxodisulphate salts, U.S. Pat. No. 4,310,394 may be mentioned, in which a cation exchange membrane is used in a two-space electrolytic cell. With the aid of a cation membrane the travelling of peroxodisulphate ions to the cathode can be prevented more effectively than with a porous diaphragm typically used.
Along with the development of ion exchange membranes the regeneration of sodium sulphate, and therethrough also of other alkali metal sulphate salts has become state of art technology by the use of combined electrolysis and electrodialysis in a three-space cell, where the middle space has been separated from the anode space with an anion exchange membrane and the cathode space from the middle space with a cation exchange membrane. Typically in such process, sodium sulphate solution is supplied into the middle space, and the products, typically sulphuric acid and lye, can be recovered from the anode space and the cathode space. While conducting direct current through a cell such as described above, the sodium ions fed into the middle space move through the cation exchange membrane into the cathode space and the sulphate ions through the anion exchange membrane into the anode space.
The method and the electrolytic cell described above have been applied in producing lye and sulphuric acid, for instance, in the FI patent application No. 911401. Using the process described in said application, about 27% lye and 40% sulphuric acid can be produced at 80% current efficiency when the thermal energy developed in the course of the electrolysis is utilized in evaporating the water in a vacuum evaporator. The electrolytic cell operates preferably in the range from +70.degree. to +150.degree. C.
U.S. Pat. No. 5,089,532 discloses a method in which lye and ammonium sulphate are produced in a three-space electrolytic cell divided with anion and cation exchange membranes. In the method, ammonia is supplied into an anode space solution. The ammonia neutralizes the hydrogen ions formed with the anode in association with the oxygen development reaction, and as a product from the anode space, ammonium sulphate is obtained instead of sulphuric acid.
U.S. Pat. No. 3,884,778 discloses a method for producing lye and hydrogen peroxide by electrodialyzing sodium sulphate in a three-space cell with a solution of sulphuric acid and persulphuric acid as the anolyte. Hydrogen peroxide can be prepared by hydrolysing persulphuric acid produced in the anode space. According to said patent, the concentration of the sulphuric acid in the anolyte is typically over 80%.
In electrolyses in which sulphuric acid is prepared in the anode space of a three-space cell described above the problem is typically the dilutedness of the product acid. This is due to the fact the anion exchange membranes currently produced are capable of retaining efficiently the hydrogen ions in the anode space merely at relatively low concentrations. As regards the other cations, the anion exchange membranes act far more successfully. Along with increasing sulphuric acid concentration, the hydrogen ions pass through the anion exchange membrane into the middle part of the cell, and from there onwards into the cathode space, whereby the current efficiencies related both to production of sulphuric acid and production of lye will be low in a continuous-action electrolysis.
By neutralizing with ammonia the hydrogen ions produced in oxygen development, the current efficiencies related to the electrolysis can be increased because the penetration of the ammonium ions through the anion exchange membrane is far less than that of the hydrogen ions.
Since the production of hydrogen ions in association with the oxygen development reaction reduces the current efficiencies of the electrolytic cell, it is sensible to act so that no hydrogen ions are formed in an anode reaction. By selecting appropriate conditions for electrolysis, the oxidation of sulphate ions into peroxodisulphate ions is the main reaction in the anode. The approach is highly advantageous compared with the processes in which oxidation forms the anode reaction.
In U.S. Pat. No. 3,884,778 mentioned above, the anode reaction is the oxidation of sulphate ions but since the anolyte is a solution of highly concentrated sulphuric acid and persulphuric acid, penetration of hydrogen ions through the anion exchange membranes currently produced cannot be avoided. This results in that in a continuous-action process the current efficiency in lye production reduces strongly.